1. Field of the Invention
The present invention relates to an organic EL (electroluminescence) displaying apparatus.
2. Description of the Related Art
An organic EL displaying apparatus is constituted by arranging pixels each having an organic EL element on a substrate in a matrix form. In each pixel, the organic EL element is connected in series to a transistor for driving the organic EL element (hereinafter, called a driving transistor) and a power supply line for supplying power to the organic EL element. Here, Japanese Patent Application Laid-Open No. 2003-122301 discloses a constitution of achieving a satisfactory moving image displaying characteristic by further providing in series a transistor for controlling an emission period (hereinafter, called an emission period controlling transistor) between the power supply line and the organic EL element.
Further, since the organic EL displaying apparatus is a self-emitting displaying apparatus, there is an advantage capable of securing high contrast as compared with a liquid crystal displaying apparatus. Furthermore, several kinds of organic EL displaying apparatuses constituted so that a user can switch over a high-luminance displaying mode and a low-luminance displaying mode according to a kind of image data have been developed. Incidentally, there is a constitution of achieving a low-luminance display by lowering a peak value of luminance. However, since a current-luminance characteristic of the organic EL element is not linear, a complicated system is necessary to make a gamma characteristic constant between the high-luminance displaying mode and the low-luminance displaying mode. On the other hand, U.S. Pat. No. 6,583,775 discloses a constitution of achieving a low-luminance display by shortening an emission period without changing a peak vale of luminance from that in a high-luminance displaying mode.
However, in case of performing driving to control the emission period as disclosed in Japanese Patent Application Laid-Open No. 2003-122301, there is a case where a defective display occurs by a leak current at a time when an emission period controlling transistor is off, for the following reason.
In the driving to control the emission period, a desired gradation display is achieved by emission luminance of the organic EL element in the emission period. In the organic EL displaying apparatus of a voltage write driving type, a data voltage being gradation displaying data is input as a data signal from a data line to the driving transistor of each pixel. The data voltage to be input as the data signal has a voltage value between a minimum gradation displaying data voltage and a maximum gradation displaying data voltage, thereby performing the gradation display.
Further, an emission period and a non emission period are defined by on and off states of the emission period controlling transistor. When resistance at a time when the emission period controlling transistor is off is not sufficiently large, a leak current flows in the organic EL element even in the non emission period in the driving sequence, whereby the organic EL element emits light. When the emission luminance (also, merely called the luminance hereinafter) by the leak current is larger than the luminance in the emission period at the time of the minimum gradation display, light emission which is larger than the luminance in the emission period at the time of the minimum gradation display is superposed in the non emission period. Thus, there is a problem that a defective display such as a luminance variation, black floating at the time of the minimum gradation display, or the like occurs.
The above problem becomes more conspicuous in the constitution, as disclosed in U.S. Pat. No. 6,583,775, of achieving the low-luminance display by shortening the emission period, for the reason that a proportion of the non emission period in the one frame period becomes long. Thus, in this constitution, since a leak emission amount to be superposed further increases, the contrast deteriorates.